1. Field of the Invention
The present invention relates to the use of a specialized horizontal tube heat exchanger operating as either a high rate falling film evaporator, a biological/chemical reactor or a gas-liquid contactor while submerged within a body of heated liquid. Principle applications for the device are (1) a solar still submerged within a salt gradient solar pond; (2) a waste heat evaporator submerged within a hot industrial effluent; (3) a biological/chemical reactor submerged within a temperature controlled liquid heating or cooling medium; and (4) a heat assisted gas stripper. Operating as a solar still, the invention may be used to purify seawater, brackish water, freshwater containing unusually high fouling impurities and for the recycling of irrigation water. Operating as a waste heat evaporator, the apparatus uses hot wastewater as an energy source to purify raw feedwater and to generate heat or vapor for industrial space heating or process uses. As a reactor the present invention has specialized application for reactions requiring careful temperature control, heat recovery, gentle mixing, and prompt removal of gaseous reaction products. As a gas liquid contactor the present invention may be used to separate or strip dissolved gases and volatile organic contaminants from water and other liquids. When used in combination with low boiling point heat transfer fluid and a Rankine cycle turbine generator system, the apparatus may be used to produce electrical power.
2. Description of the Prior Art
The horizontal tube falling film evaporator has been in commercial desalination service since the late 1960s, in the single effect vapor compression mode, and more recently, in the multiple effect configuration using generated steam as an energy source. This evaporator design has also been used in connection with solar ponds and waste heat with the intent to produce power. In order to improve the working quality of the available heat these systems have sometimes utilized low boiling point heat transfer fluids in combination with Rankine cycle turbine generators.
However, in each such case the evaporator consisted of a conventional shell and tube arrangement situated external to the heat laden liquid body. In the recent past, solar ponds have been used to heat water flowing through submerged tubing for space heating purposes without severe corrosion problems.
A rotating disk distillation device is described in U.S. Pat. No. 3,764,483 which operates on the principle of a hydrodynamically applied thin film relying on the wiping action of a flexible blade. A multiple effect version of this device has been proposed for use with a salt gradient solar pond. However, the device was proposed to be situated external to the pond and is not suitable for submerged operation, nor is the evaporation principle the same as the present invention, i.e., wiped film disk versus horizontal tube falling film.
Accordingly, there appears to be no disclosure in the prior art of an evaporator submerged within the heat source, with exposed tube sheets, so as to reduce construction costs, reduce ambient heat loss, eliminate pumping of heating medium, and that rotates to (1) maximize the falling film heat transfer effect, (2) repeatedly expose the concentrate to heat transfer surface, (3) periodically submerge a large proportion of the heat transfer tubes to reduce scaling and (4) induce flowthrough of the heating medium. This combination of features cooperate to maximize the efficiency of the evaporation process while minimizing space requirements compared to the prior art.
The art of producing drinking water from salt water by means of solar distillation is well known. Most designs of solar stills consist of floating or semi-submerged apparatus enclosed by material transparent to solar radiation wherein a pool of salinous water is allowed to heat up and vaporize.
Condensation of the vapor is frequently accomplished by routing the vapor through a deeper level of seawater. Attempts have been made to improve the efficiency of solar energy collection by using lens and solar tracking devices. Examples of prior art are disclosed in U.S. Pat. Nos. 4,325,788, 4,276,122, 4,219,387, 4,172,767, 3,986,936, 3,703,443, 3,408,260, 3,357,898 and 4,151,046.
Systems have heretofore been constructed wherein flat plate solar collectors feed hot water to conventional multistage flash distillation plants. A demonstration scale system of such a system has been in operation in Mexico for several years. Another approach has been to use photovoltaic cells to generate electricity that operates a membrane separation desalination process.
Most of the prior art dealing with solar energy collection ponds, consists of articles published in various trade and scientific journals and, in particular, recent research conducted and published on salt gradient solar ponds by the United States Departments of Interior, Energy and Agriculture. In U.S. Pat. No. 4,110,172 an enclosed solar collection pond is disclosed containing salinous water with a depth of one to ten inches. This invention is focused on the rapid heating of the shallow pool of water and the recirculation of the heated air in order to raise the efficiency of the process.
A ditch similar in configuration to the solar pond utilized in the present invention is described in U.S. Pat. No. 4,141,798. However, the solar still arrangement as described therein is substantially similar in principle to the above mentioned prior art which uses a flexible transparent plastic enclosure operating on a relatively shallow pool of salinous water, i.e., the greenhouse effect.
Applicant is not aware of any prior art which uses a salt gradient solar pond, with its superior temperature elevation capability and heat storage capability in the absence of insolation and/or in the cold season, in combination with a submerged evaporator device specially designed to take advantage of this stored heat in a space much smaller than required by shallow, non-salt gradient ponds, and constructed in such a way as to minimize the consequences of the corrosive environment.
In general, biological or chemical reactions can take place within any form of open or enclosed container. The prior art is replete with reactor configurations where the primary objective is to exercise control of the reaction to most efficiently produce the desired end product. In the biological category, reactors are generally classified as aerobic or anaerobic and suspended growth or fixed film. Most of these reactors are designed for stationary batch or continuous operation wherein nutrient laden liquids flow in and the products of reaction flow out. Solid residues are removed as necessary to maintain optimum efficiency of the process. Temperature control of a biological or chemical reaction may be accomplished by preheating or cooling the influent, by internal electrical heating elements, by heat transfer tubing within the reactor for heating or cooling, or perhaps most frequently by heating or cooling jackets on the periphery of the reactor vessel so as to avoid interference with the requirement for mixing or agitation. The aeration and/or mixing function may be accomplished by external agitation of the reactor vessel, conventional submerged blade mixers, submerged air lifts or diffused bubble aeration. Thus reactions have traditionally been maintained with respect to temperature and mixing by various combinations of the aforementioned techniques within stationary vessels.
In the late l950s fixed film rotating biological contactors were introduced in Europe and later in the United States. This type of reactor is partially submerged within the nutrient laden liquid body and its rotation is designed to allow attached organisms periodic access to both oxygen in the air and nutrients in the liquid. This apparatus consists of open media attached to the rotating support shaft and has no role in temperature regulation of the process. The rotating biological contactor has also been operated in the anaerobic mode by its complete submergence in the nutrient laden liquid body which in turn is contained in a stationary enclosed vessel. Heat is added to the process by any of the aforementioned conventional techniques.
Applicant is not aware of any prior art which uses a reactor consisting of a horizontal tube heat exchanger, submerged within a liquid heating or cooling medium, with exposed tube sheets that rotates to (1) induce flow through of the heating or cooling medium for temperature regulation, (2) periodically expose the reactor liquor to the heat transfer tubing thereby imparting a gentle mixing effect, and (3) create a thin film of liquor on the tubing from which gaseous products of the reaction may more easily separate. The entire reactor vessel embodiment of the present invention thus rotates within the heating or cooling medium, and contains the reaction liquor, rather than being submerged within it, as is the case with the aforementioned rotating biological reactor.
The present invention may be utilized as a gas-liquid contactor by introducing a gas into the heat exchanger module. Gas-liquid contacting is a mass transfer process whereby dissolved gases and volatile organic contaminants (VOC's) are separated from liquid substrates. Heretofore this process has been optimized by maximizing the concentration gradient across the gas liquid interface and by presenting a contacting surface characterized by a thin liquid film and the maximum practical surface area density. The most common device currently in use is the packed bed air stripping tower in which contaminated liquid is introduced at the top and where counter current flow of clean air is introduced at the bottom. The air is removed at the top of the tower and usually sent for destruction of entrained contaminants in an incinerator or catalytic oxidation unit or for capture of contaminants in a gas phase activated carbon filter.
A number of rotating gas-liquid contactors are described in the prior art. The common principle of these devices is to produce extremely thin films, low pressure loss and high volumetric mass transfer coefficients by contacting liquids and vapors in a centrifugal field. In recent years surface area density has been increased by adding packed media to the rotating element. These devices are designed to operate at up to 2000 revolutions per minute.
The present invention represents an improvement over the aforementioned gas-liquid contactors because of its ability to add heat directly to the gas-liquid interface. Increasing the temperature significantly improves the efficiency of the gas separation or volatile contaminant removal process. In the latter case, for example, the efficiency of contaminant removal depends on (1) the air/water ratio applied to the system, (2) Henry's Law constant of the contaminant and (3) the rate of mass transfer. The rate of mass transfer, in turn depends on the contaminant's water diffusion coefficient and the interface area. Increasing temperature positively affects two of these key process variables. Henry's Law constant increases by a factor of two to three for each 10 degrees C rise in temperature, and the water diffusion coefficient of a substance is directly proportional to the absolute temperature K.
The rotation of the present invention is not intended to create a significant centrifugal field as in the case of other rotating gas-liquid contactors. Its rotation generally between 40 and 120 RPM is intended to simultaneously achieve pumping of the heating medium and assured wetting of the contact surfaces which are the horizontal tubes within the heat exchanger. The applicant is not aware of any prior art where heat is introduced directly to the rotating, horizontal tube, gas-liquid contacting surface as in the present invention.